Abstract

The equilibrium of fluid within the body is determined by Starling’s principle, which states that hydrostatic pressure, oncotic pressure and blood vessel integrity determine the way in which fluids move within the body. An abdominal effusion develops when one or more of these three determinants is abnormal. This chapter looks at clinical presentation, diagnostic approach and treatment. Quick reference guide: Abdominocentesis.

5.1.1Small amounts of abdominal fluid can be seen with ultrasound examination. The liver lobes are separated by a small amount of free fluid (arrowed) within the abdomen, just ventral to the gall bladder. (Courtesy of North Downs Specialist Referrals.)

5.1.4Diagnostic approach when the abdominal effusion is a transudate. a Initial urine dipstick examination may indicate significant protein loss in the urine. Interpretation of urine dipstick protein must take into account concentration of urine. For example, a 1+ dipstick protein in very concentrated urine (USG>1.050) is less likely to indicate significant proteinuria than a 1+ dipstick protein in less concentrated urine (e.g. USG 1.020). Ultimately, the urine protein:creatinine ratio (UPC ratio; requested on submitted urine sample) gives the clearest indication as to whether there is significant proteinuria and should always be obtained if proteinuria is suspected. To cause hypoproteinaemia and subsequent free abdominal fluid, urine protein loss must be significant (UPC ratio >5 typically and often >10; normal is ≤0.4). A sediment examination will need to be done concurrently to rule out significant inflammation or infection within the urinary tract, as this will also cause proteinuria. b In cats it is extremely rare to develop an abdominal effusion alone if the underlying cause is cardiac failure. Cats will often develop pleural effusion or pulmonary oedema before developing an abdominal effusion. c Specialist advice should be sought if gastrointestinal biopsy specimens are to be obtained during surgery in a hypoalbuminaemic cat, due to the potential increased risk of complications.

5.1.5Diagnostic approach when the abdominal effusion is a modified transudate. In the feline patient it is extremely rare to develop an abdominal effusion alone if the underlying cause is cardiac failure. Cats will often develop pleural effusion or pulmonary oedema before developing an abdominal effusion.

5.1.6Diagnostic approach when the abdominal effusion is an exudate. PMNs = polymorphonuclear leucocytes. a When only extracellular bacteria are observed on cytology, the clinician needs to consider whether external contamination during sampling is responsible. If bacteria are intracellular, however, this signifies the presence of a septic peritonitis. Degenerative changes in neutrophils also raise suspicion of a septic peritonitis. Fluid should be submitted for bacterial culture (both aerobic and anaerobic) and susceptibility testing. b Radiography is usually not helpful if there is a significant amount of free abdominal fluid, as the loss of serosal detail precludes any meaningful interpretation. Free gas in the abdomen is an indication of either intestinal perforation or an infection with gas-forming bacteria, such as some clostridial species.

Examples of the appearance of different types of abdominal effusion. (a) Modified transudate. (b) Chyle. (c) Exudate due to feline infectious peritonitis; note the typical yellow colour. This fluid was frothy when shaken and very viscous.

Examples of the appearance of different types of abdominal effusion. (a) Modified transudate. (b) Chyle. (c) Exudate due to feline infectious peritonitis; note the typical yellow colour. This fluid was frothy when shaken and very viscous.Examples of the appearance of different types of abdominal effusion. (a) Modified transudate. (b) Chyle. (c) Exudate due to feline infectious peritonitis; note the typical yellow colour. This fluid was frothy when shaken and very viscous.

/content/figure/10.22233/9781910443149.chap5_1.ch5fig6

Abdominal fluid smear from a cat with septic peritonitis due to gastrointestinal tract perforation. All the cells visible are neutrophils (PMNs) and show evidence of a disrupted cellular outline and coarsened, ragged nuclear chromatin consistent with the hydropic change expected in degenerate neutrophils. Many rod-shaped bacteria are present over the whole of the smear and also within PMNs (following phagocytosis), confirming the septic nature of this effusion. (Diff-Quik stain; original magnification x 1000)

Abdominal fluid smear from a cat with septic peritonitis due to gastrointestinal tract perforation. All the cells visible are neutrophils (PMNs) and show evidence of a disrupted cellular outline and coarsened, ragged nuclear chromatin consistent with the hydropic change expected in degenerate neutrophils. Many rod-shaped bacteria are present over the whole of the smear and also within PMNs (following phagocytosis), confirming the septic nature of this effusion. (Diff-Quik stain; original magnification x 1000)Abdominal fluid smear from a cat with septic peritonitis due to gastrointestinal tract perforation. All the cells visible are neutrophils (PMNs) and show evidence of a disrupted cellular outline and coarsened, ragged nuclear chromatin consistent with the hydropic change expected in degenerate neutrophils. Many rod-shaped bacteria are present over the whole of the smear and also within PMNs (following phagocytosis), confirming the septic nature of this effusion. (Diff-Quik stain; original magnification x 1000)